Hydraulic cylinders typically consist of a cylinder barrel, in which a piston connected to a piston rod moves back and forth. The barrel is closed on each end by an end cap and by a gland where the piston rod comes out of the cylinder. The piston has sliding rings and seals. Typically, the piston divides the inside of the cylinder in two chambers, the expansion chamber and the retraction chamber.
In a single acting cylinder, hydraulic fluid is supplied to the expansion chamber, which applies a force against a first side of the piston causing it to move thereby increasing the size of the expansion chamber. To retract the cylinder, an external force is applied against the piston rod that causes the hydraulic fluid to exit from the expansion chamber.
In a dual acting cylinder, hydraulic fluid is supplied to the expansion chamber to extend the piston as with the single acting cylinder. For retraction, hydraulic fluid is supplied to the retraction chamber which applies a force against a second side of the piston causing it to retract. In a dual acting cylinder, the force supplied by the hydraulic fluid is dependent on the pressure of the hydraulic fluid and the surface area of the piston in contract with the hydraulic fluid. As a result of the two sides of the piston having similar surface areas and a common source typically being used to supply fluid for retraction and expansion, the ability of the cylinder to expand and contract against a load is often quite similar.
While the expansion and retraction abilities of a dual acting cylinder are often quite similar, it is common for substantially more power to be needed in the expansion direction as compared to retraction. When hydraulic cylinders are used on front end loaders, forces from gravity often assist in the retraction of the cylinder.
Attempts have been made by others to create hydraulic cylinders with asymmetric expansion and retraction power, however these designs typically involve a cumbersome additions to the cylinder and require a substantial increase in cylinder size.
U.S. Pat. No. 1,788,298 issued to Hottel in 1928 describes a fluid press or jack with an expansible chamber . . . a second expansible chamber between the inner face of the piston head and the ring or abutment . . . . For the purpose of simultaneously admitting fluid-pressure into the expansible chamber and the expansible chamber, a connection communicates with the chamber and a passageway is formed in the wall of the combined piston and cylinder structure. The Hottel design requires an extremely thick cylinder wall and is not practical for situations where the weight of the cylinder is a consideration.
U.S. Pat. No. 3,563,136 issued to Valente in 1971 describes a variable force hydraulic press with conduit means provided in said housing to achieve a fluid path from each said chamber to the exterior thereof, said fluid paths being associated with a source of fluid under pressure and control means. Accordingly, a selective application of fluid pressure to either or both said chambers may be attained, with the force exerted by said ram varying according to the total piston surface area subject to said fluid pressure. Like the Hottel patent, the Valente patent describes a press that requires a substantial amount of material and is not practical for situations where the weight of the hydraulic cylinder is a consideration.